(a) Field of the Invention
The present invention relates to a heat-sensitive recording sheet, and particularly it relates to a heat-sensitive recording sheet which is composed of a support and a heat-sensitive layer formed thereon, said heat-sensitive layer consisting essentially of a color-developable substance (developer) which is normally colorless or light-colored and an organic acid substance capable of making said color-developable substance develop a color when heated and also containing a specific wax.
(b) Description of the Prior Art
With the recent social need for increase in the amount of information, economy of resources, labor-saving, prevention of public nuisance, etc., utilization and improvement of varieties of recording systems are in progress in the field of the information recording art. Especially, the heat-sensitive recording system has merits such that (i) it can dispense with the developing process as it can develop a color upon heating, (ii) the relevant apparatus is easy to assemble and maintain, (iii) the heat-sensitive recording sheet for use therein resembles ordinary papers in feel and is rather moderate in cost, etc., and therefore it has been put to practical use as the printer part for computer outputs, portable electronic computer, etc., the recorder part of metrological instruments for medical care, facsimile, automatic booking machine, heat-sensitive copying apparatus, etc.
As the heat-sensitive recording sheet useful for these purposes, there are known such ones as disclosed in Japanese Patent Publication No. 4160/1968, Japanese Patent Publication No. 14039/1970, etc., but these heat-sensitive recording sheets in the prior art still have defects such as follows:
First, since they are inferior in heat-responsiveness, they cannot bring about a sufficient recording density adapted to the speed-up of the recording. Second, the heat-sensitive composite contained therein is fused at the time of recording and sticks to the thermal pen or thermal head, and as a result, inconvenience is caused in feeding the recording paper, the sticking substance is transferred to the recording paper to cause the so-called "tailing" phenomenon, and moreover, accumulation of the sticking substance lower the recording density, entailing indistinctness of the recorded image.
These properties are collectively called "head-matching" and constitute the most important factor to be taken into account at the time of examining a heat-sensitive recording sheet. There have admittedly been proposed various means for improving these properties, such as disclosed in Japanese Patent Open No. 19231/1973, Japanese Patent Publication No. 27599/1976, etc. That is, Japanese Patent Open No. 19231/1973 disclosed that the use of a wax having a melting point in the range of from 40.degree. to 100.degree. C. as carrier can enhance the heat-responsiveness. And, Japanese Patent Publication No. 27599/1976 disclosed that joint use of a higher fatty acid and a petroleum wax can improve the heat-responsiveness and alleviate the sticking of fused composite onto the thermal head.
However, as a matter of fact, the means disclosed in these literatures still leave something to be desired in order to obtain a fully satisfactory heat-sensitive recording sheet. In other words, from the view point of ensuring the recording fidelity and ameliorating the indistinctness of the recorded image arising from the sticking of fused composite onto the thermal pen at the time of high-speed recording by a thermal pen recorder, such as an electrocardiograph, etc., in which a tip-type pen made of a thermal head type resistor in now taking the place of a metal rod pen, a satisfactory result can, in fact, not be expected from only the arts disclosed in these literatures.